Seasonal prevalence and histopathology of Beauveria bassiana infecting larvae of the leopard moth, Zeuzera pyrina L. (Lepidoptera: Cossidae)

The natural epizootic potential of the fungus, Beauveria bassiana, against larvae of the leopard moth, Zeuzera pyrina L. (Lepidoptera: Cossidae) was investigated under natural conditions in two different locations for two seasons (2015–2016 and 2016–2017) in Rasheed, Behiara Governorate, Egypt. In an orchard, located in the old cultivated land, Beauveria infected 1.9–5.6% of larvae of Z. pyrina in autumn (September to November), increased to 3.4–7.2% in winter (December to February). The infection did not exceed 3.1% in spring (March to May), and no infection was recorded in summer (June to August). While in an olive farm, located in a newly reclaimed land, in both seasons of survey, no infections were recorded from May to September, while in winter, the epizootics ranged from 1.4–3.1%, from 0.4 to 1.3% in spring, and from 0.3 to 2.1% in autumn. Histopathological analysis revealed that Beauveria most commonly penetrated larvae directly through the intersegmental integument and the first symptoms were observed 3 to 4 days later. Hyphae radiated inward from the point of ingress and the fungus developed sparsely within the body cavity prior to the death of the larva. Hyphal bodies were concentrated around the point of ingress, but some were found at distance from this site. Only in the vicinity of penetration, the hypodermis showed signs of histolysis. The saprophytic phase developed rapidly and the fungus invaded all internal organs. The conidiophores seemed to rupture the cuticle by mechanical pressure and emerged all over the host cadaver, with sporulation occurring 48 h later. Pathological changes in the hemolymph appeared to be the primary cause of larval mortality.